1. Field of the Invention
The present invention relates generally to semiconductor laser control, and more specifically to an apparatus and method for programmable control of laser diode modulation and operating point.
2. Background of the Invention
Laser diodes and laser diode modules are increasingly used in computer and telecommunications networks, due to their low cost and wide bandwidth potential. In particular, laser diodes are used in applications such as Dense Wave Division Multiplexing (DWDM) systems. DWDM systems use laser diode outputs at multiple wavelengths. The outputs are combined and introduced into a single optical fiber to achieve data rates higher than possible with an optical connection with a single source and detector.
In order to manufacture optical transceivers at low cost and high volumes, it is desirable to produce a complete integrated circuit solution for controlling a laser diode for use in constructing a complete transceiver circuit. A complete integrated solution has not been practical, due to the variations between individual laser diodes from a single manufacturer and differences between laser diode designs from different manufacturers. Additionally, laser diodes in common use include a monitor photodiode optically coupled to the laser diode. The optical coupling of the monitor diode to the laser diode is also variable, compounding the difficulties of manufacturing a complete integrated circuit solution.
The efficiency of the laser diode and the operating point and transition times of the laser diodes vary widely, causing systems manufacturers to incorporate tuning circuits in their systems that must be adjusted after assembly of the control circuit and the laser. The resources required to tune each of the laser diode assemblies increases the cost of production and reduces the volume of components that may be produced.
Circuits incorporating electrical alterable memories have been implemented that control the bias current of the laser diode, but this is not a complete solution to the problem. Due to the variations in efficiency between laser diodes and variations in transition time for the signals transmitted by the system must be compensated via tuning.
Additionally, due to the wide variations in efficiency, a circuit designed for a low efficiency laser diode may drive a high efficiency diode at too high an AC amplitude, causing the high efficiency diode to fall out of lasing mode and entering the light-emitting diode (LED) region of operation. If the A/C modulating signal amplitude is so high as to cause the laser diode to leave the lasing mode, data communications will be completely disrupted, as the data signal is a high-frequency intensity modulation superimposed on the operating point intensity of the laser. When the laser diode falls into LED mode, the intensity drops dramatically. The loss in intensity will cause a detector that is detecting the modulation to experience a total loss of signal.
Finally, operation of the laser diode must be controlled in a manner that is stable over temperature variations, has startup characteristics that will not damage the laser diode, and will not harm personnel that may be exposed to the light emitted from the laser diode. IEEE standard 802.3 sets forth guidelines for operation of laser diode communications modules and limits on output intensity for safety of personnel.
Therefore, it would be desirable to provide a method and apparatus for programmable control of laser diode modulation swing and transition time as well as laser diode operating point to accommodate a wide variety of laser diodes from various manufacturers using a single integrated circuit component design. It would further be desirable to provide a method and apparatus that control laser diode operating point that is temperature compensated and limits the minimum and maximum output of the laser diode.
The above objective of programmably controlling laser diode modulation and operating point are achieved in an apparatus and method. The apparatus and method may set a limit control on an amplifier in the modulation signal path as well as a coupling capacitance that affects the transition times of the modulation signal as applied to the laser diode. The limit control and coupling capacitance are set by values retrieved from a memory. Additionally, a bias circuit providing a precision voltage reference and thermal compensation may be employed to provide stable control of laser diode operating point. The voltage reference may be controlled by an under-voltage lockout circuit and power-on reset circuit and supplied to an external pin of an integrated circuit in order to provide reference and control functions from a single external pin.